On Mon, Sep 28, 2015 at 10:12:37PM +0200, Paolo Bonzini wrote:
>
>
> On 28/09/2015 07:38, Haozhong Zhang wrote:
> >
> > -static u64 __scale_tsc(u64 ratio, u64 tsc)
> > -{
> > - u64 mult, frac, _tsc;
> > -
> > - mult = ratio >> 32;
> > - frac = ratio & ((1ULL << 32) - 1);
> > -
> > - _tsc = tsc;
> > - _tsc *= mult;
> > - _tsc += (tsc >> 32) * frac;
> > - _tsc += ((tsc & ((1ULL << 32) - 1)) * frac) >> 32;
> > -
> > - return _tsc;
> > -}
>
> This is basically
>
> return mul_u64_u64_shr(ratio, tsc,
> kvm_tsc_scaling_ratio_frac_bits);
>
> except that Linux has no mul_u64_u64_shr function, only mul_u64_u32_shr.
>
> We should implement that function in include/linux/math64.h instead.
> For the x86_64 case (or any other CONFIG_ARCH_SUPPORTS_INT128
> architecture) we can just write it directly, as is done already for
> mul_u64_u32_shr.
>
> For the 32-bit case, here is an implementation of both the
> multiplication and the shift, lifted from QEMU:
>
> static inline void mul64(uint64_t *lo, uint64_t *hi,
> uint64_t a, uint64_t b)
> {
> typedef union {
> uint64_t ll;
> struct {
> #ifdef __BIG_ENDIAN
> uint32_t high, low;
> #else
> uint32_t low, high;
> #endif
> } l;
> } LL;
> LL rl, rm, rn, rh, a0, b0;
> uint64_t c;
>
> a0.ll = a;
> b0.ll = b;
>
> rl.ll = (uint64_t)a0.l.low * b0.l.low;
> rm.ll = (uint64_t)a0.l.low * b0.l.high;
> rn.ll = (uint64_t)a0.l.high * b0.l.low;
> rh.ll = (uint64_t)a0.l.high * b0.l.high;
>
> c = (uint64_t)rl.l.high + rm.l.low + rn.l.low;
> rl.l.high = c;
> c >>= 32;
> c = c + rm.l.high + rn.l.high + rh.l.low;
> rh.l.low = c;
> rh.l.high += (uint32_t)(c >> 32);
>
> *lo = rl.ll;
> *hi = rh.ll;
> }
>
> static inline void rshift128(uint64_t *lo, uint64_t *hi, int n)
> {
> uint64_t h;
> if (!n) {
> return;
> }
> h = *hi >> (n & 63);
> if (n >= 64) {
> *hi = 0;
> *lo = h;
> } else {
> *lo = (*lo >> n) | (*hi << (64 - n));
> *hi = h;
> }
> }
>
> and you can easily reuse this code in Linux with just uintNN_t types
> changed to uNN + some extra cleanups when it's placed in a single functions.
>
> Paolo
Thanks! I'll add mul_u64_u64_shr() and replace __scale_tsc().
- Haozhong
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